The skull drawing from your imagination was really cool!
I suggest more intense shading contrast? Ebony really helps with that!

And for your digital art, I'm no expert, but I read this tutorial on uniformed color palettes and I think it helped a bit. It makes the colors seen not as "default", if you get what I'm saying. Ah, and I used to have really straight lines like you do (no messy sketchiness). I don't know if it's preference, but I find using many messy lines easier for sketching.
Oh hey! I'm around your age!
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